The present invention relates to a method for separating chips from a diamond wafer that comprises a substrate, a polycrystalline or single-crystalline diamond layer formed on the substrate by chemical vapor deposition, and a plurality of microelectronic elements formed on the diamond layer.
There is a method in which chips are separated by cutting a diamond wafer comprising a substrate, a chemically vapor-deposited diamond, and a plurality of microelectronic elements. However, the cutting (hereinafter referred to as xe2x80x9cdicingxe2x80x9d) of the chemically vapor-deposited diamond with a diamond saw blade cannot be performed stably because the dicing is conducted by the same substance.
To solve this problem, engineers have devised various methods in which chips are separated from a diamond wafer by using a laser together with another method such as dicing. One of the methods has been disclosed by the published Japanese patent application Tokukai 2000-21819. FIGS. 3(a) to 3(c) illustrate this method. In FIG. 3(a), a diamond wafer 101 comprises an Si substrate 102, a chemically vapor-deposited diamond layer 103 formed on the substrate 102, and a plurality of microelectronic elements 1041 and 1042 formed on the diamond layer 103. The method for separating chips is illustrated in FIGS. 3(b) and 3(c). First, as shown in FIG. 3(b), the Si substrate 102 is half-cut by a method such as dicing to form a groove. The groove has a depth less than the thickness of the substrate 102 and more than half the thickness of the substrate 102. Next, the bottom surface 105 of the half-cut groove is irradiated with a laser beam to cut the remaining part of the substrate 102 and the diamond layer 103 with a width narrower than that of the foregoing groove. FIG. 3(c) shows separated chips each having a single microelectronic element.
Another published Japanese patent application, Tokukaihei 10-125958, has disclosed another method as shown in FIGS. 4(a) and 4(b). FIG. 4(a) illustrates a wafer 111 comprising a sapphire substrate 112, an n-type nitride semiconductor layer 113 laminated on the substrate 112, and a p-type nitride semiconductor layer 114 laminated on the n-type layer 113. Chips are separated by the method shown in FIG. 4(b). First, front-side dividing grooves 115 are formed such that they penetrate through the semiconductor layers and into a part of the sapphire substrate 112. Next, back-side dividing grooves 116 having the width W2 narrower than the width W1 of the front-side dividing grooves 115 are formed on the sapphire substrate 112 at the positions aligned with the centers of the front-side dividing grooves 115. Finally, the wafer 111 is divided into individual chips having a single microelectronic element by the application of mechanical stresses.
Yet another published Japanese patent application, Tokukaishou 56-6451, has disclosed another method as shown in FIGS. 5(a) and 5(b). FIG. 5(a) illustrates a silicon-on-Insulator (SOI)-type semiconductor device 121 comprising a sapphire substrate 122, a single-crystalline silicon layer 123 formed on the substrate 122 by the vapor phase growth method, and semiconductor elements 124 formed in the silicon layer 123. As shown in FIG. 5(b), scratches 125 are formed on the surface of the silicon layer 123 with a diamond cutter. Next, grooves 126 are formed on the sapphire substrate 122 with a laser beam at the positions corresponding to the foregoing scratches 125. Finally, the SOI-type semiconductor device 121 is divided into individual chips by the application of mechanical stresses.
The method disclosed by Tokukai 2000-21819 has a processing step in which the total thickness of the chemically vapor-deposited diamond layer is cut by the irradiation of a laser beam. Consequently, a prolonged processing time is required in order to protect the microelectronic elements from thermal damage. The method disclosed by Tokukaihei 10-125958 has a processing step in which the total thickness of the semiconductor layers and a part of the thickness of the substrate are processed. The cuttings produced by the step may cause problems by adhering to the surface of the microelectronic elements. Similarly, the method disclosed by Tokukaishou 56-6451 produces cuttings during the laser processing. The cuttings may contaminate chips by adhering to them.
In order to solve the above-described problems, the present invention offers a method for separating chips from a diamond wafer having a chemically vapor-deposited diamond layer and microelectronic elements, with the microelectronic elements protected from thermal damage and degradation caused by the thermally decomposed cuttings produced during the processing steps.
In the present invention, chips are separated from a diamond wafer comprising a substrate, a polycrystalline or single-crystalline chemically vapor-deposited diamond layer formed on the substrate, and a plurality of microelectronic elements formed on the diamond layer. The chip separation is performed by the following steps: (1) Front-side grooves are formed on the chemically vapor-deposited diamond layer by laser processing using a laser such as a yttrium-aluminum-garnet (YAG), CO2, or excimer laser each having a large output so that the grooves can have a depth {fraction (1/100)} to 1.5 times the thickness of the diamond layer. (2) The thermally decomposed cuttings produced during the laser processing are removed by using a plasma. (3) Backside grooves are formed on the substrate by dicing such that the back-side grooves are in alignment with the front-side grooves. (4) The diamond wafer is divided into individual chips by applying mechanical stresses.
It is desirable that the depth of the back-side grooves formed on the substrate by the foregoing dicing be such that the remaining thickness of the substrate is at most 100 xcexcm.
When the depth of the laser processing of the chemically vapor-deposited diamond layer is less than the thickness of the diamond layer (i.e., when the laser processing remains within the thickness of the diamond layer), an oxygen gas is used for producing the plasma for removing the thermally decomposed cuttings.
When the depth of the laser processing of the chemically vapor-deposited diamond layer is at least the thickness of the diamond layer (i.e., when the laser processing is extended into the substrate) and when the substrate is made of Si or SiC, the plasma for removing the thermally decomposed cuttings is produced by using an oxygen gas, one of the CF-family gases that are CF4, C2F6, CHF3, and C4F8, and a mixed gas of oxygen and one of the CF-family gases.
When the depth of the laser processing of the chemically vapor-deposited diamond layer is at least the thickness of the diamond layer and when the substrate is made of sapphire, AlN, or GaAs, the plasma for removing the thermally decomposed cuttings is produced by using one of the CH-family gases that is, CH4 or C2H6 or one of the Cl-containing gases, that is, BCl3 or CCl4.